Materials for Civil and Construction Engineers CHAPTER 4Aluminum

Aluminum  Primarily used for containers, packaging, aircrafts, and automobiles.  In civil projects, primarily used for architectural and finishing elements like doors, windows, and siding with a small amount used for electrical wiring.  Not used extensively for structural members:  expense  strength and ductility  coefficient of thermal expansion 2

3 h 2x x W b In many beam design problems deflection is a limiting criteria. Assume a rectangular simply supported beam, and the height of the beam is fixed by other design considerations, determine the difference in width required for an aluminum beam compared to a steel beam. The equation for the deflection in a beam is: Using the subscripts a and s for aluminum and steel respectively, and using equal deflection for the aluminum and steel beam: Which reduces to: For a rectangular section the moment of inertia is: By substitution and h a = h s : Since E s = 29E6 and E a = 10.5E6 B a = 2.76 B s

Aluminum Advantages  Most plentiful metal on earth  One-third the density of steel  High strength-to-weight ratio  Good thermal and electrical conductivity  Anodizing or hard coating for protection  Weldable alloys  Easy to recycle  Corrodes slightly but does not rust  High reflectivity  Can be die cast  Easily machined  Nonmagnetic  Nontoxic 4

Chapter 4: Aluminum5 Aluminum Production Mamlouk/Zaniewski, Materials for Civil and Construction Engineers, Third Edition. Copyright © 2011 Pearson Education, Inc.

Crushing and Grinding Four tons of bauxite are required to produce 2 tons of alumina.

Extraction Al(OH)3 + Na+ + OH- ---> Al(OH)4- + Na+ Böhmite and Diaspore: AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+ Ores with a high Gibbsite content can be processed at 140 ° C. Böhmite requires °c. (At 240°c tl the pressure is approximately 35 (atm)) Although higher temperatures are often theoretically advantageous there are several disadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor. After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible through filters before being transferred to the precipitators. The insoluble mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process.

Calcination "Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina: 2Al(OH)3 ---> Al2O3 + 3H2O The calcination process must be carefully controlled since it dictates the properties of the final product. Precipitation Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor: Al(OH)4- + Na+ ---> Al(OH)3 + Na+ + OH- This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary or fluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

Filtration-Hydration Filtration, such as vacuum drum or pressure filters, remove the silica and low solids from the clarified alumina bearing liquor. The liquid containing the dissolved alumina is pumped to tanks called crystalization or precipitation tanks. The liquid is cooled with water from the counter current decantation thickeners, and as it cools, alumina hydrate slowly precipitates from the tank, according to the formula: The precipitated liquor containing the white alumina is then filtered, to remove the solid alumina from the liquid, using vacuum drum filters or rotary pan filters, where it can be washed as it is filtered. The alumina hydrate (AlOH 3 ) is then dried. It can be further calcined to Al 2 O 3, alumina, in a rotary kiln at 800 defrees F.

10 Cathode Block Molten Aluminium Feeder Gases Anode Electrolyte Anode Carbon 1.7 – 2.1 t CO 2 eq/t Al IAI average = 2.0 Electricity Input 15.6 MWh/t Al 0 – 20.8 t CO2/t Al IAI average = 5.8  GHG from Primary Aluminium Production Two PFC (perfluorocarbon compounds - CF 4 and C 2 F 6 ) contribute about 40% of direct primary aluminium GHG emissions Alumina Production 1.5 – 2.5 t CO 2 eq/t Al IAI average = 1.9 PFC Generation 0.02 – 24.5 t CO 2 eq/t Al Global average = 1.26 Source: IAI Life Cycle Inventory Data IAI 2003 PFC Survey GHGs from Primary Aluminium Production

Designation System for Aluminum Alloys 11

Stress-Strain Properties of Aluminum 12

Tensile Strength of Aluminum 13